Frequently a wizard's patronus and his animagus form are the same animal, taking the shape of the wizard's "inner animal". This, however, is not always the case. Although a person's "default" patronus is the same as their animagus form, a patronus can be changed through love to take a form that represents either a person's true love or another person (such as a parent) that gives the wizard a strong sense of safety or which represents the wizard's happy thought.

An Animagus retains his or her clothing, spectacles, and so on throughout the transformation, disappearing when the Animagus is in animal shape but reappearing when human shape is resumed. In some cases items that are always worn, such as spectacles, or body modifications, such as a tattoo, may affect the animal's appearance.

An animagus is required, by law, to register their animal form and distinguishing at the Improper Use of Magic Office in the Ministry of Magic. The penalties for not registering is an unknown sentence in Azkaban. The main reason for registration is for tracking and ensuring that each user is law abiding and not abusing their abilities.

An animagi's transformation is not a sure way to prevent detection from outside sources and there is at least one known 'reversal spell' which manifests as a bright blue light that forces an animagus to resume human shape. It is not thought to have any effect on animals neither is it known if more than one wizard is required to cast the spell.

Nothing is known for sure but it can be assumed that the act of becoming an animagus in an individual process that varies from individual to individual. In all cases, it is difficult, dangerous, and time consuming. On average, it takes a potential animagus two to four years of intensive study and preparation to become an animagus, and a wizard who is not exceptional would likely take even longer if they succeeded at all.

Once a potential animagus is ready to actually become an animagus, the danger is at its peak. One cannot practice becoming an animagus -- the magic involved is all or nothing. As such, the first transformation is critical. Not only is it when you discover what your animal form is, but it is also when the potential animagus may fail ... and failure can be a lot nastier than just "nothing happens". Some of the possibilities include death, becoming permanently stuck between human and animal forms, and being permanently stuck as an animal.

If a wizard successfully becomes an animagus, he or she has no control over what his form will be and no way to know before completing the transformation. The animagus's form is determined by their personality, and it cannot be changed once it's established. Whatever animal best represents the new animagi -- be it a centipede, a lion, or a house cat -- is forever their only animagus form.